We plan to study membrane permeability to water, other small molecules, and ions by NMR relaxation. We have already made a detailed investigation of the permeability of red blood cells to water, and plan to extend our measurements to vasopressin sensitive epithelial cells to elucidate the changes which occur in the membrane following exposure to vasopressin. We also plan to study other small molecules such as CO2, urea, ethanol and acetamide.